Add schema compatibility probe to prevent silent migration failures (bd-ckvw)

- Implement comprehensive schema probe in sqlite.New() that verifies all
  expected tables and columns after migrations
- Add retry logic: if probe fails, retry migrations once
- Return clear fatal error with missing schema elements if probe still fails
- Enhance daemon version gating: refuse RPC if client has newer minor version
- Improve checkVersionMismatch messaging: verify schema before claiming upgrade
- Add schema compatibility check to bd doctor command
- Add comprehensive tests for schema probing

This prevents the silent migration failure bug where:
1. Migrations fail silently
2. Database queries fail with 'no such column' errors
3. Import logic misinterprets as 'not found' and tries INSERT
4. Results in cryptic UNIQUE constraint errors

Fixes #262

Amp-Thread-ID: https://ampcode.com/threads/T-0d7ae2c0-9f12-4f9b-85d1-1291488af150
Co-authored-by: Amp <amp@ampcode.com>

internal/rpc/server_routing_validation_diagnostics.go

@@ -56,11 +56,23 @@ func (s *Server) checkVersionCompatibility(clientVersion string) error {
 			clientVersion, ServerVersion)
 	}
 
-	// Compare full versions - daemon should be >= client for backward compatibility
+	// Compare full versions - daemon must be >= client (bd-ckvw: strict minor version gating)
+	// This prevents stale daemons from serving requests with old schema assumptions
 	cmp := semver.Compare(serverVer, clientVer)
 	if cmp < 0 {
-		// Server is older than client within same major version - may be missing features
-		return fmt.Errorf("version mismatch: daemon %s is older than client %s. Upgrade and restart daemon: 'bd daemon --stop && bd daemon'",
+		// Server is older than client - refuse connection
+		// Extract minor versions for clearer error message
+		serverMinor := semver.MajorMinor(serverVer)
+		clientMinor := semver.MajorMinor(clientVer)
+		
+		if serverMinor != clientMinor {
+			// Minor version mismatch - schema may be incompatible
+			return fmt.Errorf("version mismatch: client v%s requires daemon upgrade (daemon is v%s). The client may expect schema changes not present in this daemon version. Run: bd daemons killall",
+				clientVersion, ServerVersion)
+		}
+		
+		// Patch version difference - usually safe but warn
+		return fmt.Errorf("version mismatch: daemon v%s is older than client v%s. Upgrade and restart daemon: bd daemons killall",
 			ServerVersion, clientVersion)
 	}
 

internal/rpc/version_test.go

@@ -323,7 +323,7 @@ func TestVersionCheckMessage(t *testing.T) {
 			serverVersion: testVersion100,
 			clientVersion: "1.1.0",
 			expectError:   true,
-			errorContains: "daemon 1.0.0 is older than client 1.1.0",
+			errorContains: "client v1.1.0 requires daemon upgrade",
 		},
 		{
 			name:          "Compatible versions",